1. Field of the Invention
The present invention relates to wide bandwidth pulsed microwave transmitters and receivers, and more particularly to short-range, sub-nanosecond pulse, phase-coherent K-band radars.
2. Description of Related Art
Range measurement of close-range targets is of great interest to a number of industries. Automotive backup warning radar, fluid level sensing in tanks and vats, material level sensing in silos, safety systems, home "do-it-yourself" projects, and aids to the blind are but a few of the applications for short-range non-contact range measurement. Radar range measurement appears to be the technology of choice but has yet to make significant penetration into these markets. The main shortcoming with radar has been the difficulty in realizing a low-cost short-pulse radar with a narrow antenna beam.
A potentially suitable radar is ultra-wideband (UWB) radar, for example, ground penetrating radar as disclosed in U.S. Pat. No. 4,698,634 by Alongi or Micropower Impulse Radar (MIR) in U.S. Pat. No. 5,774,091 by McEwan. UWB radar emits suitably short pulses (&lt;Ins) but has serious drawbacks; its low frequency spectrum can create interference with countless other spectrum users below 3 GHz, and its low frequency spectrum (i.e., long wavelength) prevents narrow antenna beam formation with a compact antenna.
Ultrasound is a potential technology that is both simple and inexpensive. Unfortunately, it is of limited accuracy since the speed of sound varies 10% over outdoor temperatures. Accuracy is of central importance in tank level measurements and construction applications, and 10% accuracy is simply not consistent with modern requirements. Accuracies of 1% to 0.01% are needed. These accuracies can be met with pulse-echo radar using precision timing techniques as will be described herein.
In addition to limited accuracy, ultrasound is susceptible to extraneous acoustic noise, and water or dirt overcoatings on its transducers can disable it. In spite of these limitations, ultrasound has been a popular ranging technology due to its simplicity and its ability to form a narrow beam with a small transducer. A narrow beam is needed to reduce clutter reflections from off-axis objects, such as a tank wall. A narrow beam also implies high antenna gain, which improves signal to noise (S/N) ratio.
While both limited and antiquated, ultrasonic rangefinding remains the dominant non-contact range measurement technology since there have been no real alternatives. One might consider an optical approach to rangefinding, such as a laser rangefinder or a video system. However, optical systems also lack environmental ruggedness--the optics cannot be located behind a decorative panel and can be disabled by an overcoating of water, snow, ice or dirt. Clearly, a better technology is needed.
Radar rangefinders are environmentally rugged: the speed of light (at which radar waves travel) does not vary with temperature (for all practical purposes), and radar waves propagate freely through wood walls, gypsum walls and plastic panels, even with an overcoating of water, ice, snow or dirt.
Pulse-echo radars operating in the 24 GHz band have a wavelength of 12.5 mm, which is almost exactly the same wavelength as 25 KHz ultrasound. Since antenna beamwidth is determined by the wavelength to antenna aperture ratio, radar and ultrasound will have comparably narrow beamwidths with the same antenna/transducer footprint.
An ultrasonic rangefinder may typically transmit a burst of 12 sinusoidal cycles of acoustic energy with a corresponding pulse width that defines the two-object resolution of the system. Of course, its incremental resolution is not a function of emitted pulse width, but that of the timing system. A 24 GHz radar with the same two-object resolution as the 12-cycle ultrasound system needs to transmit a 12 cycle, 0.5-nanosecond sinusoidal burst at 24 GHz, since the wavelengths are comparable. Clearly, the radar needs to have a wide bandwidth, on the order of 1-2 GHz.
Prior art pulse echo radars do not exhibit the combination of 1) K-band RF operation, e.g., 24 GHz, 2) sub-nanosecond RF pulse width, 3) extreme phase coherence (&lt;10-picoseconds for the entire transmit-receive system, 4) expanded time output with ultrasonic parameters, 5) simple assembly with low cost surface mount technology (SMT) components, and 6) commercially appealing size and cost. Clearly, a new technology is needed.
Attempts by the present inventor to develop a 24 GHz radar rangefinder using SMT components were met with frustration and failure--a quarter wavelength at 24 GHz is 3 mm or even less when material dielectric constants are included. Since SMT components have dimensions on the order of 3 mm, wavelength effects are a severe limitation.
One approach to counter the effect of diminishing wavelength is to decrease component size with monolithic technology such as GaAs MMIC (monolithic microwave integrated circuit). Unfortunately, the high cost of GaAs MMIC, about $10 per chip, puts radar in an uncompetitive position relative to ultrasound, which can be fully implemented on a single low cost silicon chip. A pulse-echo radar system with transmit and receive MMICs, and support circuitry might cost $50 to manufacture, after factoring-in expensive assembly techniques for very small high bandwidth components and special circuit board materials. In contrast, a complete ultrasound system can be manufactured for under $5.